In an engine provided with a catalytic converter, it is known that supplying secondary air into the housing of the converter during the warm-up period of the engine may accelerate the activation of the catalyst in the converter.
For example, in Jikkai Sho 64-22816 published by the Japanese Patent Office in 1989, it is disclosed how a secondary air supply inlet is provided in an exhaust passage connecting the catalytic converter with an engine exhaust manifold, and part of the air aspirated into the engine intake passage is directly supplied to this inlet without the intervention of the engine. As the catalyst is not active immediately after startup, this supply of secondary air to the exhaust passage increases the oxygen concentration in the interior of the catalytic converter, thereby accelerating temperature rise of the catalyst and promoting its activation.
When the catalyst is activated, the supply of secondary air is stopped. The determination of whether or not the catalyst has become active is made by determining, for example, whether or not the engine cooling water temperature has reached a predetermined temperature.
When the air-fuel ratio of the gas mixture supplied to the engine is feedback-controlled, catalyst activation is an important factor. In this case, the time required for activation depends also on the cooling water temperature at engine startup. Tokkai Hei 1-280651 published in 1989 and Tokkai Hei 7-71304 published in 1995 by the Japanese Patent Office, for example, therefore propose that a feedback control start timing should be determined according to the cooling water temperature at engine startup. Engine cooling water temperature has thus conventionally been widely employed as a parameter for determining catalyst activation.
However, as catalyst activation and cooling water temperature do not have a unique correspondence, a precise determination of activation cannot be made simply by determining whether the cooling water temperature has reached a predetermined temperature. The determination of catalyst activation is also not precise if it is determined by the elapsed time after engine startup for which different reference values are set depending on cooling water temperature at startup, because the determination takes no account whatever of engine running conditions after startup.
In any of these cases, there may therefore be a considerable time error between the activation as determined and real activation, and this error leads to various disadvantages.
Specifically, when the supply of secondary air is stopped before catalyst activation is completed, the completion of catalyst activation takes a longer time.
Conversely when the supply of secondary air is continued after the catalyst is activated, reduction of nitrogen oxides (NOx) in the exhaust is prevented by the excess oxygen.
Moreover, the catalyst will be overheated which will lead to its early deterioration.
When air-fuel ratio control assuming that the catalyst has become active, is performed before full activation, toxic components in the exhaust increase. The same is true when air-fuel ratio control assuming that the catalyst is not activated, is continued after full activation.
Hence, insofar as far as concerns secondary air or air-fuel ratio control, errors in determining catalyst activation are undesirable whether they are positive or negative.